Heterologous immunity occurring as a consequence of T cell cross-reactivity between unrelated pathogens has been shown by us in animal models to contribute to either enhanced or reduced viral loads and remarkably altered immunopathology against vaccinia virus, arenaviruses, influenza virus, and cytomegalovirus. Examples of such heterologous immunity have now been reported in human infections with Epstein-Barr (by us), dengue and hepatitis C viruses. The overall objective of this grant is to exploit our developed mouse models to determine how T cell frequencies, functions, and repertoires change as the host undergoes successive acute or persistent infections with non-identical viruses, much like that which occurs in humans. Insights on these issues are necessary for the intelligent design of modern vaccines that are effective and without unwanted side effects, as unanticipated T cell cross-reactivity can sometimes lead to damaging pathology. Many issues of heterologous immunity are still unexplored. We know little about how the altered cross-reactive repertoire is formed and how it contributes to immunopathology. We know little about the impact of heterologous immunity as the T cell receptor (TCR) repertoire narrows in the aging host or in hosts rendered lymphopenic by infection, cancer, or immunosuppressive drug treatment. We also lack insight on why heterologous immunity between different pathogens is not necessarily reciprocal. To clarify these issues we propose to examine changes in the TCR repertoire and affinities of cross-reactive T cells under conditions of sequential and persistent viral infections, to examine the MHC basis of heterologous immunity to confirm that it is mediated by TCR-MHC interactions, to examine heterologous immunity under conditions of limited TCR repertoires caused by age or lymphopenia, to determine how frequencies and functions of naive bystander T cells are influenced by infection, and to shed insight on the lack of reciprocity in protective heterologous immunity between pathogens.